Phenol-aldehyde/bentonite adhesive composition and wood lamination process

ABSTRACT

Disclosed is a new adhesive and a process for making wood laminates with the adhesive. In a preferred embodiment, the adhesive will comprise a phenol-formaldehyde condensation product having a molar ratio of phenol to formaldehyde in the range of 1:1.5 to 1:2.5, a sodium hydroxide catalyst, and from 5 to 50% bentonite clay based upon the phenol-formaldehyde condensation product solids. The preferred process employs the new adhesive to prepare laminates using high moisture content veneers, such as southern pine. The laminates exhibit average percent wood failures of from 85 to 98%.

BACKGROUND OF THE INVENTION

The present invention relates to a new adhesive and a process for makingwood laminates employing the adhesive.

A wide variety of adhesives and processes are known for preparing woodlaminates. Among the most widely used adhesives are variousphenol-aldehyde condensation products. Typically, these adhesives areapplied to wood plies and the plies are then subjected to a pressingoperation to consolidate them into a unitary panel and cure theadhesive. In many situations it is desirable to perform the pressing intwo or more stages to maximize the output from a fixed number of heatedplaten presses. Typically, a first pressing stage will be performed atambient temperature under a pressure and for a time just sufficient toconsolidate the separate plies into a unitary panel. A panelconsolidated in this fashion can be stored and handled without shiftingor separation of the plies. The consolidated panels are then typicallycured under heat and pressure to form the final laminate.

The use of phenol-aldehyde adhesives is quite common in the productionof wood laminates. In U.S. Pat. No. 3,637,547 to Orth, a colloidal,acicular magnesium aluminum silicate clay, such as attapulgite andsepiolite, is added to an otherwise conventional phenol-aldehydeadhesive. These particular clays are added in amounts of from 5 to 50%based on the weight of the phenol-aldehyde solids. In Example I of theabove patent, panels prepared with an adhesive including 45% bentoniteclay had a dry wood shear failure of 50% and a wet wood shear failure of35%, as compared to values of 90-100% and 90-95% for identical samplesprepared with attapulgus instead of bentonite.

It has generally been preferred in the past to prepare wood laminatesfrom essentially dry wood plies because moist veneers invariably producepanel blows on opening the hot press after panel curing. Moreover,adhesives of the phenol-aldehyde type cure by condensation and aretherefore inhibited to a certain extent by the presence of water.Recently, however, more attention has been given to the production oflaminates using wood plies, such as southern pine veneers, havingmoisture contents of from 0 to 14% or higher based on the dry weight ofthe wood. The use of high moisture content plies is advantageous becauseconservation of energy due to reduced drying time and therefore moreeconomical. Unfortunately, conventional adhesives do not performsatisfactorily in this environment, and usually prolonged pressing timesare incurred and an undesirable number of laminates must be rejected dueto panel blows.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new adhesivecomposition.

It is another object of the present invention to provide a new adhesivecomposition suitable for making wood laminates.

It is another and more specific object of the present invention toprovide a new adhesive composition especially suitable for making woodlaminates from high moisture content plies.

It is another object of the present invention to provide an improvedprocess for making wood laminates employing a new adhesive compositionhaving excellent tackability.

It is yet another and more specific object of the present invention toprovide an improved process for making wood laminates from high moisturecontent plies using a new adhesive composition.

These and other objects are achieved according to the present inventionwhich provides a new adhesive composition and an improved process formaking wood laminates. The new adhesive composition comprises: aphenol-aldehyde condensation product having a molar ratio of phenol toaldehyde in the range of 1:1.5 to 1:2.5, an alkaline catalyst selectedfrom the group consisting of alkali metal and alkaline earth metalhydroxides; and from 5 to 50% of bentonite clay based on the weight ofcondensation product solids. The process comprises: (a) applying the newadhesive to mating surfaces of a plurality of wood plies in an amount offrom 70 to 90 pounds per square foot of double glue line; (b) assemblinga panel of the plurality of wood plies by mating the surfaces to whichthe adhesive has been applied; (c) pressing the assembled plies underpressure for a time effective to create a bond of sufficient strengthbetween them to hold them together in a unitary, consolidated panel; and(d) pressing the consolidated panel under conditions of pressure,temperature and time effective to cure the adhesive.

DETAILED DESCRIPTION OF THE INVENTION

The new adhesive composition of the present invention has a wide rangeof utilities, and can be employed for most of the same applications thatphenol-aldehyde condensation product adhesives have been used in thepast. Thus, they may be used not only in the production of woodlaminates from dry and or high moisture content wood plies, but also forthe manufacture of hardboard made of wood particles, molded articlescontaining fabric reinforcement, and a wide variety of otherapplications. Likewise, the process of the invention is not limited inutility to making wood laminates, from high moisture content wood plies,although this would be the preferred embodiment. For the purposes ofconciseness, however, the following detailed description will bedirected to the preferred embodiment of preparing a wood laminate fromhigh moisture content plies and especially high moisture content woodveneers.

The preparation of wood laminates from high moisture content plies andveneers is becoming increasingly important due to the need to conserveenergy whenever possible. Because wood plies used to make laminatestypically have moisture contents of from 3 to 5%, based on the dryweight of the wood, a high moisture content wood is defined for thepurposes of the present description as one with a moisture content ofgreater than 5% and less than 19%. The usual and preferred range formoisture content will be from about 5 to about 14%. The term "ply" asused herein shall mean any one thin sheet of wood forming one or a partof one layer in a multi-layered, laminated wood product.

Typical of the varieties of wood which are readily available with highmoisture content are: soft woods such as southern pine, Douglas fir andthe like, and hard woods such as birch, walnut and the like.

The new adhesive compositions of this invention comprise aphenol-aldehyde condensation product, a suitable alkaline catalyst andfrom 5 to 50%, based on the weight of the condensation product solids,of bentonite clay. Preferably, the bentonite clay will be employed in anamount of from 20 to 35%. Bentonite clay is essentially sodiummontmorillonite. It is derived from the Pacific Coast states, swellswhen wetted, and has strong adsorbing properties. One suitable form ofcommercially available bentonite clay is sold by Georgia Kaolin Companyunder the trademark Bentonite and Bentolite.

While hydroxybenzene-formaldehyde condensation product is most oftenused as the phenol-aldehyde adhesive, other aldehydes and phenolsgenerally used for adhesives may be employed. Illustrative of some ofthe aldehydes which may be used are the aliphatic aldehydes such asacetaldehyde and propionaldehyde; aromatic aldehydes, such asbenzyladlehyde and furfural; and such other aldehydes as aldol, glyoxal,and crotonaldehyde. Illustrative of some of the phenols which may beemployed in place of hydroxybenzene are resorcinol, cresol,pyrocatechol, cresylic acid, xylenols, naphthols, such as polyphenolssuch as the bis-hydroxyphenoly alkanes as 2,2'-bis-(4-hydroxyphenyl)-propane. The molar ratio of phenol to aldehydeused can be varied from 1:1.5 to 1:2.5, preferably in the range of1:1.8-1:2.5.

Also in the formation of the adhesive, other constituents usuallyemployed in phenol-aldehyde adhesives used for wood laminations may beadded to impart the beneficial characteristics normally obtained by theparticular constituents, along with the bentonite clay being used astaught herein. Necessarily, an alkaline catalyst, preferably selectedfrom the group consisting of alkali metal and alkaline earth metalhydroxides, is used to catalyze the condensation of the adhesive.Generally, the amount of the hydroxide used is in a molar ratio ofhydroxide to phenol in the range of from 0.4:1 to 0.9:1. Preferably, atleast one filler selected from the group consisting of starch, woodflour, nutshell flour, bark products or agriculture residues, and wheatflour is employed in the adhesive in amounts of from 1 to 10 weightpercent of the resin solids.

According to the process, the new adhesive of the invention is appliedto the mating surfaces of a plurality of wood plies. The surfaces whichwill be on the exterior of the panel are preferably not coated.

The adhesive is applied to the plies generally in amounts from 70 to 90pounds, preferably from 70 to 80 pounds, per square feet of double glueline. After assembly, the panel is subjected, at ambient temperature,typically being from 15° to 30° C., to a pressure sufficient to bringthe plies into contact with each other to consolidate the plies.Generally, a pressure in the range of 150 to 200 pounds per square inchis used. The plies are subjected to the pressure until a sufficientinitial or pre-press bond is obtained to keep the plies from separatingor coming apart upon the release of the pressure. A bond of the requiredstrength is usually obtained in a contact time in the range of 0.5 to 5minutes, although in a particular type of panel construction asatisfactory bond may be obtained in the time required to attain contactpressure or the pressure necessary to bring the plies into contact witheach other.

Consolidation of the panels by pre-pressing permits the panels to behandled much more easily and quickly than one stage procedures andtherefore allows a greater extent of automation. Also, the number ofplatens loaded in the hot press can be increased, since the panels arenear to the final thickness. Less curling or fold back of the outerplies is also obtained, thus decreasing the number of rejects. In hotpressing, the panels may be subjected to the pressures and temperaturesnormally employed for heat setting of the particular phenol-aldehydeadhesive. Pressures of from 50 to 500 pounds per square inch attemperatures in the range of 90° to 200° C. are often used. The panelsare subjected to the pressing for a sufficient time to bring the panelsup to the curing temperature of the adhesive which, for the normal typeof panel construction, may usually require 1 to 20 minutes.

A feature of the present invention is that the wood laminates made withthe new adhesive exhibit vacuum pressure and boil shear test strengthgreater than would be expected for a bentonite-containingphenol-aldehyde adhesive, and superior to results which have heretoforebeen achieved with any phenol-aldehyde adhesive system for woodlaminates made from high moisture content plies. The adhesive preparedwith bentonite has excellent tackability. The vacuum pressure andboiling shear tests are standard methods for objectively rating theadhesive bonds of a wood laminate. These shear value tests are conductedby the test procedures described in U.S. Product Standard PSI-74 (Aug.1, 1974)--Section 4.5.2 for Vacuum Pressure Test and 4.5.3 for BoilingTest. Thus, V/P shear % wood failure scores are expressed inpercentages; and, in the example, a score of 95% means that the brokenarea of the shear sample contains 95% wood fibers. A one inch squaresample after shear testing contains 95% wood fibers and 5% fiber freearea. In other words, the average wood failure of all test specimens isnot less than 95%. Similarly, the boiling shear % wood failure score isdetermined in the same manner.

The following examples are presented for the purpose of furtherillustrating and explaining the present invention and are not to betaken as limiting in any regard. Unless otherwise indicated, all partsand percentages are by weight.

EXAMPLE I

This example illustrates the preparation of a wood laminate employinghigh moisture content plies including a high moisture content veneer ofsouthern pine.

The following formula was prepared:

Resin--912 parts

Water--200

Bentolite--100

Glu X--50

Co Cob--50

(50%) Caustic--50

and applied to the both faces of the 2nd and 4th plies of a five plypanel construction, at a spread rate of 85 lbs./MDGL. After the desiredassembly time of from 10-40 minutes had elapsed, the panels were coldpressed for 31/2 minutes, at a pressure of 175 psi. This was followed byplacing the panels in a hot press at a temperature of 300° F., under apressure of 180-200 psi for 7 minutes.

Two sets of plywood (Southern pine) 5 ply panels were prepared using 5%Moisture Content Veneers and 11% Moisture Content Veneers, respectively.

The Average Shear values for the above prepared panels were as follows:

    ______________________________________                                        5% Moisture Content Veneers                                                                           226 psi                                               Vacuum pressure                                                                                       90% wood failure                                                              198 psi                                               Boil testing                                                                                          91% wood failure                                      11% Moisture Content Veneers                                                                          232 psi                                               Vacuum pressure                                                                                       88% wood failure                                                              187                                                   Boil testing                                                                                          85% wood failure                                      ______________________________________                                    

When the control glue Formula was prepared (same as in Example, butusing no bentolite i.e., as follows:

Resin--912 parts

Water--200

Co-cob--150

Glu X--50

(50%) Caustic--50

and was used with the same moisture content veneers as shown above.

The average shear values for these panels were as follows:

    ______________________________________                                        5% Moisture Content Veneers:                                                                          240 psi                                               Vacuum pressure                                                                                       91% Wood failure                                                              203 psi                                               Boil testing                                                                                          92% wood failure                                      ______________________________________                                    

All of the 11% Moisture Content Veneers produced panel blows. As thebentolite was increased to 150 parts or reduced to 50 parts with theco-cob decreased or increased in the above formula. The average shearvalue results of the prepared panels using the two different percentMoisture Content Veneers, namely 5% MC and 11% MC showed similar resultsfor the 5% Moisture Content veneers, but lower average shear values forthe 11% Moisture Content Veneers. The average % wood failure for the 11%Moisture Content Veneer panel was 63 percent.

The above description is presented for the purpose of enabling thosepeople skilled in the art to make and use the present invention, and itis not intended to detail all those obvious modifications and variationswhich will become apparent upon reading. It is intended, however, thatall those obvious modifications and variations be included within thepresent invention, the scope of which is defined by the followingclaims.

What is claimed is:
 1. A phenol-aldehyde adhesive which comprises aphenol-aldehyde condensation product having a molar ratio of phenol toaldehyde in the range of 1:1.5 to 1:2.5, an alkaline catalyst selectedfrom the group consisting of alkali metal and alkaline earth metalhydroxides, and from 20% to 35% of bentonite clay based on the weight ofcondensation product solids.
 2. An adhesive according to claim 1 whereinthe condensation product is a hydroxybenzene-formaldehyde condensationproduct.
 3. An adhesive according to claim 1 wherein the molar ratio ofphenol to aldehyde is in the range of 1:1.8 to 1:2.0.
 4. An adhesiveaccording to claim 1 wherein the catalyst is an alkali metal hydroxidepresent in a molar ratio of hydroxide to phenol in the range of 0.3 to0.9.
 5. An adhesive according to claim 4 wherein the catalyst is sodiumhydroxide.